The invention relates to a phased-array apparatus, and, more particularly, this invention relates to such apparatus for providing ultrasonic scanning of an object.
In phased-array equipment, that is, an electronic sector scanner, the change of signal delay of the individiual ultrasonic transducer elements in the case of transmitting and receiving must take place in very small steps to avoid errors in the adjustment of the control angle. Due to the fact that the typical maximum control angle is generally .+-.45.degree. relative to the normal of the transducer element array, large control angles require relatively long delay times, whose length depends moreover greatly on the selected aperture length (length of the active antenna). To compensate the change of resolution with the depth because of the limited definition of the focussed aperture, it is desirable to adapt the receiving focus concomitantly.
The conventional technique provides the adjustment of the delay times by means of inductive - capacitive delays or LC delay lines which are equipped with setting taps. This relatively inexpensive solution is suitable especially for short delay times, i.e. for non-sweeping or non-deflecting, e.g. a linear array. With longer delay times the LC delay lines have a band-limiting effect for higher frequencies. They constitute, therefore, a low pass filter whose cutoff frequency may be about 5 MHz. At the same time, component tolerances greatly affect the accuracy of the entire delay. For this reason, LC delay lines for transducer frequencies are generally used only to about 3.5 MHz. This technique is referred to also as the "baseband technique."
Higher transducer frequencies can be processed with the aid of LC delay lines by down-mixing to an intermediate frequency below 3.5 MHz. The down-mixing technique, however, presupposes a constant signal bandwidth and transmitting pulse length of the individual transducer signals. But in the interest of good resolution, the transmitting pulse time length should be changed, i.e. reduced, when changing over to high transducer frequencies.
Another possible technique is provided by the surface wave filter technology of SAW filter technology (see e.g. Ultrasonics, Vol. 17, pp. 225-229, Sept. 1979). Here it is necessary to mix the received signal of the individual ultrasonic transducer element upward, so as to get into the high frequency band of 20-50 MHz required in the SAW technique. After the summation of the individual received signals of the phased-array, down-mixing is necessary. Disadvantages of the SAW technique are the fact that in each channel upwardmixers must be employed, involving considerable expense, and the problems of obtaining a sufficiently fine graduation of the delay times in the SAW filters.
Upward and downward mixing operations in connection with a phased-array equipment are kown. For example, German Patent No. 28 54 134 in FIG. 11 discloses such mixing operations. Digital delay technology in a phased-array equipment is also described in European Patent No. 0.027,618, in particular in FIGS. 1 and 2.
In the design of phased-array equipment also the following viewpoints must be considered:
If it is assumed, for example, in a medical test a center frequency of the received spectrum of f.sub.s =3.5 MHz and if we consider theoretically a band width .DELTA.f=f.sub.s (2 lambda pulse), we obtain as maximum frequency f.sub.smax =f.sub.s +.DELTA.f/2=1.5 f.sub.s =5.25 MHz. From this results, according to Shannon's theorem, a scanning frequency for the individual ultrasonic transducer element of f.sub.a &gt;2 f.sub.smax =3 f.sub.s 10.5 MHz. This scanning frequency f.sub.a, therfore, is the minimum frequency for being able to reconstruct the individual signal of a transducer element.
For the quantization of the phase, i.e. for a sufficient accuracy of the time delay between two adjacent transducer elements, scanning with at least 1/8 of the wavelength is necessary. This results in a quantized phase shift within the wavelength lambda of 360.degree./8=45.degree. or (.+-.22.5.degree.). At a center frequency f.sub.s =3.5 MHz one obtains therewith a time delay of 35.7 nsec, i.e. .+-.17.9 nsec. This accuracy of phase or time requires a scanning frequency f.sub.a &gt;28 MHz if the signal is to be processed digitally (see European Patent No. 0,027,618). This high scanning frequency currently requires the use of emitter - coupled logic or ECL components and leads to a relatively expensive phased-array equipment.
A way out of this velocity problem is the quadrature technique (cf. German Patent, N28 54 134, FIG. 8), where two delay channels phase shifted by 90.degree. are made use of. Here the minimum scanning frequency is f.sub.a =10.5 MHz. It permits the use of energy-saving techniques (e.g. HCMOS, Low Power Schottky). The quadrature technique, however, involves a relatively high expense, as it requires two channels per transducer element for signal processing.
It is the object of the invention to provide a phased array equipment which provides high accuracy in the adjustment of the control angle in an, economic way.